1. Field of the Invention
The invention relates to an image forming apparatus such as electrophotographic copying apparatus, electrostatic recording apparatus, or the like, more particularly, to an image forming apparatus for obtaining an image by transferring a visible image such as a toner image formed on an image holding body onto a recording medium held on a recording medium holding member.
Specifically speaking, the invention relates to an image forming apparatus of a recording apparatus using an electrophotographic system, ink jet system, or the like, for example, a printer, a facsimile apparatus, a copying apparatus, or the like.
2. Related Background Art
FIG. 16 shows an example of a conventional image forming apparatus to form a full-color image. According to the image forming apparatus in such an example, a photosensitive drum 1 as an image carrying member is held so as to be rotatable in the direction shown by an arrow. A corona charging device 2, an optical system 3, a developing apparatus 4, a transfer apparatus 5, and a cleaning device 6 are arranged around the photosensitive drum 1.
The optical system 3 is constructed by, for example, a laser beam exposing apparatus, having an original scan section and color separating filters, for irradiating a color separated light image or a light image E corresponding to it onto the photosensitive drum 1. By irradiating the light image E every separation color by the optical system 3 onto the photosensitive drum 1 uniformly primary-charged by the charging device 2, an electrostatic latent image of each color is sequentially formed on the photosensitive drum 1.
The developing apparatus 4 is a rotary developing apparatus and have following four developing devices around a center axis 4b: namely, a cyan developing device 4C; a yellow developing device 4Y, a magenta developing device 4M, and a black developing device 4K. A predetermined developing device is rotated and moved to a developing position which faces the photosensitive drum 1 and a latent image on the photosensitive drum 1 is developed, thereby forming a toner image by a toner using a resin as a base substance onto the photosensitive drum 1.
A recording medium is taken out from a recording medium cassette 7 and is fed by a conveying system to the transfer apparatus 5 along a paper path (conveying path) shown by a broken line in the drawing. The toner image formed on the photosensitive drum 1 is transferred onto the recording medium fed to the transfer apparatus 5 at a transfer position which faces the photosensitive drum 1.
In the example, the transfer apparatus 5 has a transfer drum 5a. The transfer drum 5a is formed by a cylindrical body in which a recording medium holding sheet (transfer sheet) 5f made of a dielectric material is suspended in a space area of an outer peripheral surface. A transfer charging device 5b, an adsorption charging device 5c, an adsorbing roller 5g, inside and outside discharging devices 5d and 5e, and a separation discharging device 5h are arranged around the transfer drum 5a.
The recording medium supplied from the recording medium cassette 7 to the transfer apparatus 5 via the conveying system is electrostatically adsorbed and held on the holding sheet 5f by the adsorption charging device 5c comprising corona charging units on the inside and outside of the transfer drum 5a and the adsorbing roller 5g arranged at a position opposite to the adsorption charging device 5c. The recording medium is subsequently conveyed toward the transfer position by the rotation of the transfer drum 5a.
At the transfer position, an electric field is applied to the recording medium by the transfer charging device 5b comprising a corona charging unit on the inside of the transfer drum 5a, whereby the toner image on the photosensitive drum 1 is transferred. The recording medium onto which the toner image was transferred is again conveyed to the transfer position by the rotation of the transfer drum 5a, then a toner image of a next color formed on the photosensitive drum 1 is transferred onto the recording medium.
When toner images of four colors are transferred as mentioned above, the recording medium is discharged by the separation discharging device 5h on the outside of the transfer drum 5a and is separated from the carrying sheet 5f by a separating means 8 and is fed to a heat roller fixing device 9. The toner images are color-mixed and fixed onto the recording medium by the heat roller fixing device 9, thereby forming a full-color permanent image. After that, the recording medium is delivered onto a tray 10. On the other hand, residual toners on the surface of the photosensitive drum 1 is cleaned and eliminated by a cleaning device 6, thereafter, the drum 1 is again used for an image forming process.
In the image forming apparatus as mentioned above, generally, a pair of registration rollers 43 as a kind of conveying rollers which can selectively rotate and stop, are provided just before an adsorbing position where the adsorption charging device 5c faces the adsorbing roller 5g. After a front edge portion of the recording medium conveyed from the recording medium cassette 7 along the conveying path reached a nip entrance portion of the pair of registration rollers 43 in a stop state, the registration roller pair 43 start to rotate at a predetermined timing and feed the recording medium to the adsorbing position.
Before the registration roller pair 43 start to rotate, a pair of conveying rollers 42 on the upstream side of the conveying path also continuously feed the recording medium toward the stopped registration roller pair 43 just after the front edge portion of the recording medium reached the nip entrance portion of the registration roller pair 43, thereby forming a loop of the recording medium between the registration roller pair 43 and the conveying roller pair 42. Even in the case where the recording medium obliquely moves before it reaches the registration roller pair 43, consequently, a whole region of the front edge portion of the recording medium collides with the nip entrance portion of the registration roller pair 43. Simultaneously with the start of the rotation of the registration roller pair 43, the front edge portion of the recording medium coincides with a direction of generating line of the registration roller pair and is fed out from the registration roller pair 43.
However, hitherto, there is a case where a conveying speed of the recording medium that is fed out from the registration roller pair 43 remarkably differs from a peripheral speed of the transfer drum 5a or a deviation occurs between the generating lines of the registration roller pair 43 and transfer drum 5a. There are consequently problems such that a fluctuation in rotation of the transfer drum 5a is caused by a conveying force of the recording medium that is fed out from the registration roller pair 43 or a distortion occurs in the recording medium carrying sheet 5f, so that a color deviation, a transfer variation, or the like occurs.
In the case where a curl control of the recording medium is performed in the conveying path on the upstream side than the conveying roller pair 42, if a loop of the recording medium which is formed between the conveying roller pair 42 and the registration roller pair 43 is formed in the direction opposite to a curling direction, there is a problem such that a curl is decreased and an effect of the curl control deteriorates or the like.
To solve the above problems, there has been proposed a method whereby after the front edge portion of the recording medium was fed out, the nip portion of the registration roller pair 43 is away from the recording medium, thereby decreasing a conveying force of the recording medium or releasing the loop.
However, particularly, in the case where a rear edge portion of the recording medium exists on the upstream side than the conveying roller pair 42, there is the following drawback. Namely, just after the front edge portion region of the recording medium starts to adsorb to the recording medium holding sheet 5f, if the nip portion of the registration roller pair 43 is away from the recording medium, a deviation occurs in the conveying direction between the front edge portion of the recording medium adsorbed to the holding sheet 5f and fed out from the registration roller pair 43 and in which the oblique movement has been eliminated and the rear edge side of the recording medium that is fed out by the conveying roller pair 42. Namely, only the front edge portion of the recording medium is obliquely fed out and the rear edge side is obliquely moved. In such a case, a defective adsorption, a transfer deviation, or the like occurs.
In the image forming apparatus of the type having a plurality of paper feed units as mentioned above, if each unit such as transfer drum, image forming section, or the like is arranged to the optimum position while realizing a compact size of the apparatus, generally, a paper path cannot help becoming a crank-like shape as shown in the drawing. A loop space in front of the registration roller pair is ordinarily provided so as to form a loop toward the inside of a curved portion R before the registration roller pair at a position near the curved portion R as shown in FIG. 17. This is because it is difficult to further provide a space toward the outside thereof since the paper is conveyed along the outside of the curved portion R.
In case of the apparatus of the type such that the recording paper is wrapped around the transfer drum and the image is transferred onto the surface thereof as mentioned above, if there is a slight floating portion or the like in the paper held on the drum, there is a situation such that an adverse influence such as change in image density, transfer blank, or the like is exerted on the transfer image. To avoid such a problem, accordingly, for example, there is used a method whereby a curling mechanism 96 for curling the paper is arranged in the conveying portion before the registration roller pair, thereby curling the paper to easily move along the transfer drum, or the like.
However, the following problems occur in the above apparatus. The curl of an amount which has been once given to the paper in the conveying portion before the registration roller pair so as to be easily moved along the transfer drum is changed by the loop before the registration roller pair. For example, in the apparatus, the curled portion becomes a loop shape as shown in FIG. 17 and the paper P' is bent three times (1), (2), (3)! while changing the direction of a curvature. Thus, the curl amount of the paper eventually decreases and when the paper is moved along the transfer drum, the front and rear edge portions of the paper easily float from the drum, so that there is a possibility such that an image blank or the like occurs. Or, a phenomenon such that the paper is peeled off from the drum is also considered in accordance with the circumstances.
To prevent such drawbacks, a method of increasing the curl amount to a value more than a necessary amount on the assumption of a change by the loop is also considered. However, it is substantially impossible to uniformly decide such a curl amount for all of the papers having different thicknesses and stiffnesses. (For example, as a stiffness of a paper is large, a change amount of the curl due to the loop is large. As a thickness of a paper is thin, such a change amount is small. Further, such a change amount is also not stable.) When a curl of an amount more than a necessary amount is given, this means that a large performance is requested to the curling mechanism, so that an efficiency is low. When further strictly explaining, since the loop before the registration roller pair is generally formed after the center portion, it doesn't uniformly exert an influence on the whole surface of the paper. The front edge portion is hardly influenced. It will be understood from the above explanation that the loop before the registration roller pair doesn't exert an influence on the curl as much as possible.
In consideration of such a point, there is also a method of reducing a curvature even in the same loop amount by widening the loop space as much as possible. However, it is difficult to assure such a loop space and it results in an increase in size of the apparatus. When the loop amount is simply reduced, there is a limit for the inherent purpose to effect a registration of the paper. Particularly, the use of a plurality of paper feed sections which is a general tendency in recent years causes an increase in length of paper path up to the registration rollers and a complication of the paper path. However, such a structure results in an increase in variation of the paper conveying timing. In order to absorb such a variation as well, a loop of a certain extent is indispensable.